It has long since been known how the occurrence of defects can be detected. The occurrence of defects in a film, for example a polymeric film, can be accomplished by means of a device which illuminates a film, for instance, from below with a light source. The film can be observed from above by a detector, for example by a line array. At the beginning and the end of the passage of a defect, the portion of the detector on which the present portion of the film is reproduced will not be equally illuminated, since the end and the beginning of the optically refracting defect temporarily break into the image of the non-radiating surface behind and in front of the light-radiating opening on the light table.
An alternative design may be that a similar device is provided with a scanning beam of light (e.g. laser light) which illuminates a film and wherein the light beam is observed from the other side by a detector, e.g. a photo-detector. This detector is arranged so that the detection only verifies if there is light passing through the film. Should no light be detected, the light beam will accordingly have been refracted so that the light beam either passes in front of or behind the detector because of the defect which is present on the film.
In order to find and analyze the defect, it can be marked with the aid of a marking device, for example of inkjet type, which marks the position of the defect on the strip by means of indicating, for example with ink, directly on the strip. The detector and the marking device can be electrically connected to each other, wherein the detector can transfer signals to the marking device in case a defect is detected. When a defect is detected, the marking device can mark this on the strip, for example by means of an inkjet printer.
One way of detecting defects in a transparent plastic film is disclosed in U.S. Pat. No. 4,038,554. The defects are measured by means of a laser beam striking a mobile mirror, wherein the mirror performs a scanning movement across the film so that the laser beam can scan the entire width of the film. A light-collecting rod is situated underneath the plastic film and is provided with a light-refracting tape in the longitudinal direction of the rod and a photo-diode on one of the short sides of the rod.
Another way of detecting defects on a transparent glass plate is disclosed in U.S. Pat. No. 3,989,387. The defects are measured by means of illuminating a glass plate with a light beam, wherein the light beam which has passed the glass plate is reflected onto a mirror. Accordingly, the light beam in the main can pass through the glass plate once more and be detected by a photo-electric cell. In order to obtain a suitable angle of reflection, the light beam is conducted through the glass at an angle up to 30.degree. in relation to a normal, which is perpendicular to the longitudinal direction of the glass.
However, these documents only disclose how it is possible to detect defect and not how a defect can be marked on a strip. Furthermore, when marking of defects with the aid of inkjet printers is concerned, certain problems may occur such as the ink running on the strip. Furthermore, inkjet printers normally require high precision and maintenance, e.g. when cleaning out ink.
One method of detecting and marking optically refracting defects in transparent glass plates is disclosed in U.S. Pat. No. 4,492,477 and is utilized for the preamble of claims 1 and 6. The defects in a glass plate can be detected by means of illuminating one side of the plate with a light source and detecting possible defects with a detector on the other side of the plate. A marking system can be arranged in connection with the detector, which can mark a defect directly on the plate. This marking system, however, only discloses how it is possible to utilize some kind of ink which is coated directly onto the plate. By means of coating the ink directly onto the plate, the ink can run on the plate. If the system is able to maintain high precision, it may be necessary to use special inkjet nozzles which require a special type of ink. The ink should be rapidly drying in order to avoid the ink running across the glass plate, which in turn contributes to another problem, namely that the ink inside the nozzles dries and, accordingly, they have to be cleaned. If this method were to be utilized for marking defects on a polymeric material, such as polyethylene or teflon, instead of glass as is disclosed in U.S. Pat. No. 4,492,477, it may furthermore be necessary to use another type of ink. This implies that the method will be difficult to apply to different types of materials, since it is not always possible to use the same type of ink.